In dental prosthesis, crowns and bridges are generally prepared from special noble metal alloys, which are covered with porcelain. In preparing such prosthesis, however, a balance must be drawn between factors such as the necessary mechanical strength of the finished product, the strength of the bond between the alloy and the porcelain coating; and, of course, the influence of the alloy on the color of the porcelain coating.
White gold alloys are known in the art for use in preparing dental prosthesis. U.S. Pat. No. 4,123,262 and U.S. Pat. No. 3,981,723 describe white gold alloys that have met with particular success in the art. Such alloys have the desirable features of:
High strength, which allows for a broad range of appliance design and construction; PA1 Ease of processing, in that the alloys can be processed without developing dark oxides upon heating, which had at one time been a problem due the presence of copper and/or cobalt. Modern alloys, by eliminating these elements avoid the previous problems encountered with dark oxides. The soldering which occurs during the joining of various components of complex prosthesis can also be accomplished without creating dark oxides, using the modern alloys; PA1 moderate melting ranges, in that the modern alloys can be easily cast by a variety of methods such as torching, heat induction or resistance heating. In addition, low melting phases which might cause distortion during porcelain firing are largely eliminated in the modern alloys.
Although the known white gold alloys provide high strength, ease of processing and moderate melting ranges; it is also desirable to avoid discoloration that results from the use of otherwise desirable elements in dental alloys. Thus, for example, silver is frequently employed as one of the elements in known alloys, but the presence of silver often leads to discoloration of the porcelain which is applied over the alloy in preparing dental prosthesis.
Some porcelains have been made to include agents to prevent the discoloration caused by the silver. This method of addressing the discoloration problem has shortcomings, in that the laboratory preparing the prosthesis must monitor the silver accumulation in the porcelain-firing furnace, since the agent used can handle only a limited amount of silver before losing its effectiveness, whereupon discoloration will appear.
Thus, it is preferred to eliminate silver wherever possible.
A further problem encountered in the use of gold alloys in the preparation of dental prosthesis is that of matching the thermal expansion of the base alloy to that of the porcelain that is applied to it. The known alloys are generally classified as having either high thermal expansion coefficients or low thermal expansion coefficients, and can be used, accordingly, with high thermal expansion porcelains or low thermal expansion porcelains. Dental laboratories have therefore found it necessary to maintain supplies of different kinds of alloys, so that the proper alloy could be used in accordance with the thermal expansion properties of the porcelain to be used.
Therefore, in addition to requirement that dental alloys not deteriorate the color of porcelain coatings during processing, thermal expansion coefficients that would enable the alloys to be used together with a wide choice of porcelains would be highly desirable
U.S. Pat. No. 4,123,262 represents a substantial advance in the art, in that it provides a dental gold alloy that completely eliminates the need for silver and thus avoids the porcelain discoloration that results from the presence of silver.
The alloys described in U.S. Pat. No. 4,123,262, however, have a thermal expansion coefficient of about 14 .mu.m/m/.degree. C. in the temperature range of 25 to 600.degree. C., and are therefore useful primarily for porcelains having a low thermal expansion coefficient. The use of this alloy with porcelains having high thermal expansion properties does not always produce the most desirable results.
U.S. Pat. No. 3,981,723 describes an alloy that has a thermal expansion coefficient of about 14.7 .mu.m/m/.degree. C. in the temperature range of 25 to 600.degree. C., and is therefore useful primarily for porcelains having a high thermal expansion coefficient, and is not desirable for use with porcelains having low thermal expansion properties. A further disadvantage of this alloy is that it includes silver in its formulation, and therefore can discolor porcelain being applied to castings made therefrom.
There is therefore a need for a dental alloy that is free of silver, and which at the same time has a thermal expansion coefficient that enables it to be used with both porcelains that have high thermal expansion properties, and those having low thermal expansion properties.
It would also be desirable if an alloy could be found which as amenable to welding by the laser-welding technique. Laser-welding machines are finding increased use in dental laboratories, but some alloys heretofore known in the art are not amenable to laser-welding. It has been found, for example, that alloys which contain tin can be difficult to weld by the laser-welding technique. The reason for this difficulty is believed to arise because, at the point where a portion of the alloy becomes molten during the welding procedure, the prevailing conditions favor the formation of tin oxide. Tin oxide, in turn, is a refractory (i.e., it is not easily reduced) and collects in the grain boundaries of the weld, rendering the resulting joint brittle.
A dental alloy which does not require the presence of tin would therefore be highly desirable.
It is therefore an object of the present invention to provide an alloy that can be used with a wide range of porcelains, while maintaining the desirable features of high strength, ease of processing, moderate melting range and a composition that does not deteriorate the color of porcelain applied thereto.
It is a further object of the present invention to provide an alloy that can be welded using a laser-welding technique, without encountering the brittleness encountered in the known tin-containing alloys.